Global demand for diesel, particularly for ultra-low-sulfur-diesel (ULSD) has risen quickly with increased growth of transportation fuels and a decrease in the use of fuel oil. Regulations for transportation fuels have been established to substantially lower the sulfur levels in diesel fuels. There are other pending rules calling to reduce the sulfur content in off-road diesel as well. Thus, there is a growing need for hydrocarbon feeds to use as feedstocks for producing diesel, including ULSD.
A refinery produces a number of hydrocarbon products having different uses and different values. It is desired to reduce production of or upgrade lower value products to higher value products. Two examples of lower value products are cycle oils and heavy hydrocarbons.
Cycle oils have historically been used as blend-stock into fuel oil. However, such oils cannot be directly blended into today's diesel fuels because of their high sulfur content, high nitrogen content, high aromatics content (particularly high polyaromatics), high density, and low cetane value.
Heavy hydrocarbon feeds contain compounds with high boiling points, and are generally characterized as having high asphaltene content, high viscosity and high density. Today, producers of heavy hydrocarbon mixtures have few options for their use, and the options available have relatively low commercial value.
Both cycle oils and heavy hydrocarbons have been used in heating oils. However, the sulfur contents of these hydrocarbons may limit their use due to recent regulations calling for more stringent heating oil sulfur standards.
Hydroprocessing, such as hydrodesulfurization and hydrodenitrogenation, have been used to remove sulfur and nitrogen, respectively from hydrocarbon feeds. An alternative hydroprocessing operation is hydrocracking, which has been used to crack heavy hydrocarbons (high density) into lighter products (lower density) with hydrogen addition. If the nitrogen content is too high in the hydrocarbon mixture going into the hydrocracking process, the zeolitic hydrocracking catalyst may be poisoned. In addition, if the hydrocracking is too severe, significant amounts of naphtha and lighter hydrocarbons, which are considered as lower value products, may be produced.
Conventional three-phase hydroprocessing units used for hydrotreating and high pressure hydrocracking, commonly known as trickle bed reactors, require hydrogen from a vapor phase to be transferred into liquid phase where it is available to react with a hydrocarbon feed at the surface of the catalyst. These units are expensive, require large quantities of hydrogen, much of which must be recycled through expensive hydrogen compressors, and result in significant coke formation on the catalyst surface and catalyst deactivation.
Alternative hydroprocessing approaches include hydrotreating and hydrocracking in a once-through flow scheme as proposed by Thakkar et al. in “LCO Upgrading A Novel Approach for Greater Value and Improved Returns” AM, 05-53, NPRA, (2005). Thakkar et al. disclose upgrading a light cycle oil (LCO) into a mixture of liquefied petroleum gas (LPG), gasoline and diesel products. Thakkar et al. disclose producing a low sulfur content diesel (ULSD) product. However, Thakkar et al. use traditional trickle bed reactors, which require large quantities of hydrogen and large process equipment such as a large gas compressor for hydrogen gas circulation. Significant amounts of light gas and naphtha are produced in the disclosed hydrocracking process. The diesel product accounts for only about 50%, or less, of the total liquid product using LCO feed.
Kokayeff, in U.S. Pat. No. 7,794,585, discloses a process for hydrotreating and hydrocracking hydrocarbon feedstocks in a “substantially liquid phase”, which is defined as the feed stream has a larger liquid phase than a gas phase. More specifically, hydrogen may be present in a gas phase up to 1000 percent of saturation. Kokayeff teaches such high amounts are needed so that as hydrogen is consumed, hydrogen is available from the gas phase. Thus, Kokayeff's reaction system is a trickle bed. Separation of gases occurs after hydrocracking and before recycling a portion of the liquid product. Thus, hydrogen gas is lost from the reactor effluent, which may be significant, as Kokayeff teaches adding hydrogen well above the hydrogen saturation limit of the liquid.
It is desirable to have a process for hydroprocessing hydrocarbon feeds in a smaller and simpler system without an added gas phase or gas separation that may result in loss of process hydrogen. It is also desirable to have a process for hydroprocessing hydrocarbon feeds to produce low sulfur diesel in good yield and achieving multiple desirable diesel properties such as low density and low poly-aromatic content and high cetane number. It is further desired to have a process to upgrade lower value refinery hydrocarbons to higher value products.